Ultraviolet light (UV) is electromagnetic radiation with a wavelength of approximately 4 to 400 nm (nanometers).  These wavelengths are outside the region normally detected by the human eye, and therefore are considered to be invisible. 

Ultraviolet light has been used since the early 1900's in Europe for the disinfection of municipal water supplies.  UV is currently receiving renewed attention as a method of wastewater disinfection.  A major reason for this renewed interest is that UV adds nothing to the wastewater during the disinfection process. 

UV disinfects by altering the DNA of the bacterial cells exposed to it.  It has been found that UV radiation with a wavelength of approximately 254 nm is most efficient for disinfection purposes. 

In practical use at wastewater treatment plants, UV light is produced by low pressure mercury lamps.  These lamps which provide radiation of 253.7 nm, are usually housed in specially fused quartz sleeves.  Glass sleeves cannot be used as glass absorbs UV light with great efficiency.  This would result with no or little UV light reaching the wastewater;  therefore, very little disinfection would occur.  The quartz sleeves serve as an electrical insulator by preventing the wastewater from contacting the electrical portion of the lamp and also as a temperature buffer so the bulb may remain at its optimum operating temperature for maximum efficiency. 

The usual configuration is for the mercury lamps to be in a closed unit.  This is done for safety and to promote complete mixing.  The closed unit contains baffles which direct the wastewater flow so that the microorganisms will spend a maximum amount of time close to the ultraviolet source. 

For the ultraviolet disinfection process to be effective, the UV radiation must be directed on the bacteria.  The UV unit therefore attempts to expose all bacteria to the radiation at a reasonably close range (i.e. 1.4 inch).  To achieve disinfection, most bacteria require 6000 to 13000 microwatt seconds of exposure.  Commercial available UV disinfection units can provide in excess of 30000 microwatt seconds.  This high amount serves as a safety factor as the mercury discharge lamp output deteriorates with use. 

The three main disadvantages of UV are:  (1)  high cost of operation, (2)  anything which will prevent the UV light from reaching the bacteria will prevent an effective kill, and (3)  UV light tends to ionize compounds and break them apart (i.e. nitrate could become nitrite in UV light), causing toxic effects on the effluent.  Suspended solids, slime growth, turbidity, and color are some of the factors which have an adverse effect on UV disinfection. 

Better design of the disinfection units has attempted to solve some of these problems.  For example, most units have automatic wipers for the quartz tubes to control slime build-up. 

Recently, experimenters have been able to achieve effluent limits of 200 fecal coliform/100 mL in wastewater with a suspended solids content of 50 mg/L at a flow of several million gallons per day.  This efficiency is attributed to design improvements in the disinfection units. 
 
 

Safety of Ultraviolet Disinfection

Ultraviolet light poses a special problem because it is invisible.  Intense UV exposure can result in first temporary and eventually permanent damage to the eye, possibly leading to blindness.  Should it be necessary to perform work inside the UV unit, be sure it is off and remains off during the maintenance procedure.  The symptoms of intense UV exposure are the the feeling of sand in the eyes, although more is present.  Exposure of skin will result in reddening of the skin, similar to sunburn, but possibly, depending on the exposure, much more severe.